This invention relates generally to high performance laser systems. Merely by way of example, an embodiment of the present invention includes pulsed fiber laser sources that emit temporal pulses shorter than 10 ns with high peak power, narrow spectral linewidth, and arbitrary pulsetrains suitable for frequency conversion. However, the scope of the present invention is broader than this application and includes other laser systems.
In some laser systems, a master-oscillator, power amplifier architecture is utilized in conjunction with a frequency conversion stage. The master oscillator emits a periodic train of temporally short, low power pulses. In some instances these pulses may have been stretched by applying a linear chirp in order to reduce nonlinearities in the fiber amplifier. This emission is coupled into a saturated fiber amplifier that emits a pulsetrain with the same temporal shape, but at higher powers. The output of the power amplifier then enters into a frequency conversion stage that emits light of a different wavelength generated by propagation through one or more nonlinear optical media.
In order to maximize output/input power efficiency, power amplifiers are generally operated in the highly saturated regime of amplification. If the relaxation lifetime of the gain (e.g. ˜100 microseconds to several milliseconds with Yb-doped gain fiber) is much longer than the pulse period of the master oscillator input signal, the gain cannot dynamically respond to the time variations of the input signal. Consequently, the gain of the amplifier responds as though the input signal is continuous wave (CW), and the average output power of the amplifier is only dependent on the average input power from the master oscillator. In the case of very high saturation, the output power is only weakly dependent on the input average power. In practice, the input average power can be varied by as much as 10 dB with little effect on the output average power.
This saturated amplifier behavior can be used to increase the output pulse energy by reducing the repetition rate. Since the same amount of average power will be delivered at a reduced repetition rate, it follows that the pulse energy (and pulse peak power) of the output light increases as the repetition rate decreases.
A negative aspect of saturated amplification is that if the seed power is decreased or eliminated altogether, the inversion of the gain medium will increase dramatically on the timescale of the gain relaxation rate. With this higher inversion comes a higher gain, often resulting in the amplifier lasing due to weak back-reflections or large amounts of amplified spontaneous emission being generated. Both phenomena result in large amounts of power propagating in both directions in the amplifier, often resulting in damage of optical components. Furthermore, high inversion in a Yb-doped gain fiber can result in accelerated rates of detrimental photodarkening of the gain fiber.
Another deleterious effect of a sub-saturated power amplifier is its behavior when the master oscillator pulsetrain is restarted. The first several master oscillator pulses that are injected into the power amplifier are amplified by very high gain, since the inversion is higher than when the amplifier is saturated. The gain can be up to 30 dB greater than when the master oscillator is operating in saturated mode. Thus, these “leading pulses” will contain very high peak power and often result in optical damage to the amplifier or harmonic conversion stages of the laser system. Furthermore, if the inversion increases in the power amplifier, the pump absorption will decrease causing components downstream from the pumping lasers to be exposed to very high average powers. This may cause failure to these components.
Many applications for fiber lasers require the ability to arbitrarily turn pulsetrains on or off. In order to protect the laser system, the pumping power to the fiber amplifier is generally decreased or completely removed during the time that pulses from the laser are absent. This ensures that the fiber amplifier will operate only when emission pulses from the master oscillator are present. However, modulating pump lasers for the amplifier dramatically slows process work time due to thermal issues and long response times of the high power pump lasers. Other laser systems attempt to modulate the emission of continuous train of pulses and gate them after the laser with a shutter-like apparatus. At higher speeds and higher powers, this method becomes impractical. Thus, there is a need in the art for improved methods and systems for fiber lasers.